The present invention relates to a contact start plasma torch.
In torches of this type, normally and widely used for cutting metals, an electric arc, consisting of a voltaic arc formed between the metal object being worked on and an electrode, is used together with monoatomic or biatomic gas (for example containing argon, hydrogen, nitrogen, oxygen and others) to bring the gas to the plasma state with extremely high temperatures for melting the metal.
The structure of these torches normally comprises a torch body housing:
a cylindrical electrode mounted centrally on the torch body and connected by a wire to the negative pole of the power supply;
a cap or nozzle shaped anode mounted externally and surrounding the electrode, so that it covers the end of the electrode. This nozzle is electrically insulated from the electrode and can be connected, by a second wire, to the positive pole of the power supply.
The nozzle is, in turn, held in its stable operating position by another external element, called a nozzle holder, which also acts as an element connecting the various internal parts of the torch body, these parts having to be disassembled for regular substitution of parts subject to wear during use of the torch.
The nozzle holder is coated with an insulating material so that it is safe to operate and allows the gas to exit through a plurality of holes, or annular zones, distributed at its distal portion close to the nozzle-electrode assembly, to cool the operating zone.
At present, in contact start plasma torches the electrode is, in a non-operating configuration, positioned in contact with the nozzle and the electric arc is activated by moving the electrode away from the nozzle.
To move the electrode, the latter may be attached to a central shaft, housed in the torch body and mobile between the above-mentioned non-operating position in which it makes contact with the nozzle, and an operating position in which it is distanced from the nozzle, using mechanical means or the plasmagenic gas, that is to say, the gas used to generate the plasma.
Depending on the solution used for its movement, the back of this central shaft may have an architecture designed to form thrust surfaces close to the rear end, the thrust provided, for example, by the plasmagenic gas at infeed so as to allow the shaft to slide, moving the electrode away from the nozzle and so starting the plasma arc when supplied with the plasmagenic gas.
Normally, to switch off the torch, the gas supply is cut off as well as switching off the electrical power supply. A mechanical element, preferably a spring, acts on the shaft to move the electrode back towards and into contact with the nozzle.
However, this plasma torch architecture has a significant operating delay if the torch is repeatedly switched off and started, as happens during normal use.
This disadvantage is due, in particular, to the presence of gas in the pipes and the plasma chamber after switch off, gas which tends to keep the electrode away from the nozzle until completely evacuated from the internal chambers, for a finite although short time. In addition, this relative movement is not instantaneous, with inevitable wear between the parts moving relative to one another and so shortening their useful life.
The aim of the present invention is, therefore, to overcome the above-mentioned disadvantage by providing a plasma torch which has a simple, rational and safe architecture, with extremely rapid repeated start and switch off speeds and consisting of accessory elements which last extremely well over time.
This aim is achieved by a contact start plasma torch consisting of: a hollow shaft connected to a first supply pipe for a flow of a first gas and to an electrode which is hollow so that it surrounds part of the shaft and forms a first chamber for cooling the electrode and for outfeed of the first gas; a nozzle surrounding the electrode to form a second chamber for receiving the first gas for generating the plasma, and a third chamber for the passage of the first gas from the first, cooling chamber, through relative third pipes, to the second chamber through second pipes passing through the nozzle. There are also first sealing parts inserted between the shaft and the nozzle and on both sides of the third pipes, forming a sealed zone close to the third pipes; a cylinder for moving the shaft, acting on the shaft through an inflow and, respectively, an outflow of a second operating fluid in a fourth chamber of the cylinder, to provide a forward starting position, in which the electrode is in contact with the nozzle, and a back operating position, in which the electrode is distanced from the nozzle, in the presence of the first gas.